In a seat used in a plane, a train, a ship, an automobile and the like, a seat back is required to be tilted not more than necessity when a large impact force is applied upon a head-on collision or a rear-end collision. When a seat back tilts too deep, a passenger on a back seat may be injured. It is also conceivable that the leg is caught in a steering, which may cause a risk to receive an injury by a load on the waist or the like or receive an injury on the head by baggage put on the back seat. Therefore, it is required to restrain a tilt of a seat back to restrain displacement of the human body by pushing the passenger's back to a cushioning material disposed on a back frame so as to increase absorbency of impact. Hitherto, various countermeasures to increase yield strength (the backward moment strength) against such a backward rotation moment applied to the seat back have been proposed. However, most of them relate to improvement of a seat frame structure, and even when such an improved seat frame is used, development of technology capable of increasing the intensity of the backward moment has been still required.
The present invention has been made in view of the above problems and an object of the present invention is to provide a seat structure capable of increasing intensity of the backward moment more than before so that the impact resistance can be further improved even when a seat frame designed to reduce in weight is used.